1. Technical Field
This invention relates to the production and processing of steels to achieve ultrafine microstructures. For example, in a ferrite containing steel, ultrafine microstructures are considered to be those having a significant proportion of grains of a size less than 5 microns in a plain carbon steel, or less than 3 microns in a microalloyed steel.
2. Related Art
One of the principal aims of modem steel processing methods is to refine ferrite grain size. A small ferrite grain size is desirable as this results in a steel with improved strength and toughness.
In recent years, there have been several reports in the scientific literature of different techniques for producing low carbon microalloyed steels with ultra fine ferrite grains. One type of approach has relied upon the expectation that dynamic recrystallisation at temperatures only a little above the austenite to ferrite transformation temperature (Ar.sub.3) will produce a small grain size. Controlled rolling schedules have thus been devised, using laboratory simulation by torsion or compression testing, which exploit dynamic recrystallisation after strain accumulation.
In one case, Kaspar et al reported production of austenite grains down to 1 to 4 micron in a compression tested Nb-V microalloyed steel which transformed on cooling to ferrite with a mean grain size less than 5 micron ["Thermec 88" Proc.Int.Conf. on Physical Metallurgy of Thermomechanical Processing of Steels and Other Metals, I.S.I.J. 1988, 2, 713]. Samuel et al reported that torsion testing of niobium microalloyed steels produced austenite and ferrite grain sizes of 5 and 3.7 micron, respectively, in deformation schedules where strain accumulation from successive passes led to dynamic recrystallisation [I.S.I.J. Int., 1990, 30, 216].
U.S. Pat. No. 4,466,842 to Yada et al describes a hot-rolled ferritic steel composed of 70% or more of equiaxed ferrite grains having an ultra-fine grain size of 4 .mu.m or less. This steel is produced by hot working at approximately the Ar.sub.3 point and by one or more passes of hot working having a minimum required total reduction ratio of at least 75%. Due to hot working, dynamic transformation of austenite and/or dynamic recrystallisation of ferrite takes place.
For plain carbon steels, Matsumura and Yada [I.S.I.J. 1987, 27, 492 and "Thermec 88" I.S.I.J. 1988, 1, 200] disclosed hot working schedules using laboratory compression and rolling tests to produce ferrite grain sizes below 3 micron. By imposing large strains just above the Ar.sub.3, they induced transformation during the deformation (despite the increase in temperature from the heat of deformation) and then continued to work the ferrite sufficiently for it to recrystallise dynamically. Rapid quenching after the deformation, while preventing coarsening of the ferrite grains, led to some martensite formation. By imposing strains up to 4, microstructures with 70-80% ferrite as fine as 1 to 2 micron were produced. Reducing the amount of intercritical deformation tended to reduce the volume fraction of ferrite and to increase the mean grain size.
Other techniques to produce ultrafine grains have been more involved. Ameyama et al. ["Thermec 88", I.S.I.J. 1988, 2, 848] disclosed low temperature deformation and brief austenitising cycles, combined with the addition of 3% Mn and 1% Mo to enhance austenite nucleation on reheating, to produce austenite grain sizes down to 1 micron in diameter. Kurzydlowski et al. [Z. Metallkunde, 1989, 80, 469] also disclosed repeated cold deformation and anneal cycles, together with boron additions, to produce austenitic stainless steels with grain sizes down to 1 micron diameter. Although these methods are of considerable scientific interest, they are a relatively expensive means of producing ultrafine grains.
More recently, Beynon et al have reported [Materials Forum 1992, 16, 37] the production of ultrafine Nb microalloyed ferrite, with an average grain size of approximately 1 micron, using laboratory hot torsion tests. The tests utilised controlled hot deformation at a temperature of about 1050.degree. C., followed by rapid cooling through a sequence of six to eight finishing deformations, starting at 900.degree. C. Each deformation was to a strain of 0.3 at an equivalent uniaxial strain rate of 2.3/s, and the final deformation was close to Ar.sub.3, when maximum refinement was observed. The finest structure produced was a uniformly fine equiaxed ferritic microstructure with approximately 5% pearlite and a mean grain size for the ferrite of 1.3 micron. It was proposed that the refinement was due to strain induced transformation of a heavily controlled rolled initial austenite microstructure, in which the deformation increases the density of the nucleation sites for transformation to ferrite. Such a mechanism of ferrite refinement had been reported in the Matsumura and Yada paper first listed above. Priestner ["Thermomechanical Processing of Microalloyed Austenite", Met.Soc.A.I.M.E., 1981, 455] also obtained fine grains in regions of laboratory rolled samples which transformed in the roll gap during rolling. Again a large strain was necessary and the transformed product was mixed and quite "patchy", with some very large grains present. The processes reported by Beynon et al and by Priestner are again of scientific rather than practical interest.
It is a first preferred object of the invention to provide a practical process for the production of steels with ultrafine microstructures in any of a variety of phases or mixtures of phases, including eg bainite.
It is a second preferred object of the invention to provide a practical process for the production of steels with ultrafine ferrite microstructures.
It is a third preferred object of the present invention to provide a steel with an ultrafine microstructure, particularly an ultrafine ferrite microstructure.
It is a fourth preferred object of the present invention to provide apparatus for use in the production of steels with ultrafine ferrite microstructure.